1. Field of the Invention
The present invention relates to a metal oxide material, and particularly to a metal oxide material useful as a superconductive material.
2. Related Background Art
Among copper oxides, materials represented by the compositional formulas La.sub.2-x A.sub.x CuO.sub.4 (where A is Ca, Sr or Ba), YBa.sub.2 Cu.sub.3 O.sub.y and SrBiCuO.sub.y are known as superconductive materials.
However, in the conventional superconductive materials comprising copper oxides, the yttrium and lanthanoids that are component elements of the materials have been expensive because of their small estimated reserves. In instances in which these copper oxides are formed into sintered compacts or thin films in accordance with the applications, high temperatures of about 1,000.degree. C. are required as the reaction temperature, resulting in the disadvantages of a high production cost and considerable limitations on the substrates. Moreover, they can also be formed into single crystals with difficulty, and under narrow reaction conditions, so that no large crystal has ever been obtained. Furthermore, deviation in compositional ratios may greatly affect the superconductivity transition temperature (hereinafter "Tc"), causing the problem that the materials exhibit no superconductivity within a certain range, for example, at x.gtoreq.0.2 in Y.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.7. This is particularly a serious serious problem since the compositional deviation becomes liable to occur when thin films are prepared.
Relating to the material represented by the compositional formula: SrBiCuO.sub.y, Z. Phys. B-Condensed Matter 68, 421-423 (1987) discloses a material, having its composition of Sr.sub.2 Bi.sub.2 Cu.sub.2 O.sub.7+.delta., and Tc of 7 to 22K. in the midpoint. This Bi-based superconductive material does not employ any expensive starting materials such as Y and lanthanoids as its component elements, can be formed using reaction temperatures of not higher than 900.degree. C., can be inexpensive in comparison with conventional La.sub.2-x A.sub.x CuO.sub.4 and LnBa.sub.2 Cu.sub.3 O.sub.y, and can afford to accept a broader scope of selection in respect of the materials for substrates when thin films are formed, and thus can be said to be superior materials in these respects. They, however, have the Tc that tends to be extremely lowered by contamination with impurities, and it is difficult to obtain a superconductive material having a stable Tc not lower than the boiling point of liquid helium (4.2 K.), so that it is required to use starting materials purified to a high degree.
Japanese Journal of Applied Physics, Vol. 27, February, 1988, pp.L209-L210, discloses a Bi-based BiSrCaCu.sub.2 O.sub.x, and Tc of 120 K. at on-set temperature and superconductive material having its composition of 75 K. at zero-resistance temperature, and moreover suggests the possibility that the zero-resistance temperature shifts to 105 K. at the compositional ratio of Bi:Sr:Ca=1:1:1. However, in a superconductive material having the composition of BiSrCaCu.sub.2 O.sub.x, the zero resistance is not achieved at 105 K., but the zero resistance is achieved after another transition has further appeared at the lower temperature side. Even in the transition at the lower temperature side the transition temperature range is as broad as about 10 K, showing that the material has no good uniformity. This cannot bring about a sufficient reproducibility when the materials is used as a device having a fine structure.
The above superconductive material also has a lower critical magnetic field of as low as 5 Oe (oersted) at a temperature of about 55 K., and is not satisfactory for use in a magnetic shield. Thus, there remain problems to be solved.